This invention relates to an improvement in the structure of filter press solid polymer electrolyte electrolysis cells. More particularly it relates to those of such cells which employ permselective ion exchange membranes having an electrocatalytic material bonded to or embedded in the membrane and acting as anodes or cathodes. Such cells are particularly useful in the electrolysis of aqueous solutions of alkali metal chlorides; especially in the electrolysis of aqueous solutions of sodium chloride (sodium chloride brine). The cell structure may also be used in electrolyzing other solutions to make products such as potassium hydroxide, iodine, bromine, bromic acid, persulfuric acid, chloric acid, adiponitrile and other organic compounds made by electrolysis.
It is well established that various chemicals can be produced in an electrolytic cell containing an anode and a cathode. For example, alkali metal chlorates, such as sodium chlorate, have been formed electrolytically from a sodium chloride brine in cells without a separator positioned between the anode and the cathode.
When a separator, such as a liquid permeable asbestos or polytetrafluoroethylene diaphragm or a substantially completely liquid impervious ion exchange membrane, is used in a cell to electrolyze a sodium chloride brine, the electrolytic products will normally be gaseous chlorine, hydrogen gas, and an aqueous solution containing sodium hydroxide.
For a number of years gaseous chlorine was produced in electrolytic cells wherein an asbestos diaphragm was interposed between finger-like, anodes and cathodes which were interleaved together. During the past several years it has become apparent that the use of a substantially liquid impermeable cation exchange membrane may be preferable to the more well established diaphragm in instances where a higher purity, for example a lower sodium chloride content, higher sodium hydroxide product is desired. It was found to be more convenient to fabricate ion exchange type electrolytic cells from relatively flat or planar sheets of ion exchange membrane rather than to interleave the membrane between the anode and cathode within the older finger-like cells used with asbestos diaphragms.
The newer, so-called flat plate electrolytic cells using a planar piece of ion exchange membrane to separate the anolyte from catholyte compartments also have a plurality of solid, liquid impervious frames adapted to support the anode on one side and the cathode on the opposite side. These frames have previously been constructed of materials such as metal and plastic, but neither of these materials has been found to be entirely satisfactory. In any electrolytic cell, including both monopolar and bipolar cells, there is a possibility that electrolyte may leak from within the cell to the exterior. In instances where such leakage has occurred in cells with iron or other ferrous type frames, it was found that the iron frame corroded or was itself electrolytically attacked. Plastic frames are not generally subject to the electrolytic attack, but are normally not resistant to the anolyte and/or catholyte within the cell under operating conditions for extended periods of time, for example, several years.
Solid polymer electrolyte membranes consist of an ion exchange membrane having an electrically conductive, electrocatalytic material embedded in or bonded to at least one side of the ion exchange membrane. Such electrodes are well known in the art and are illustrated in, for example, U.S. Pat. Nos. 4,457,815 and 4,457,823. Solid polymer electrolyte membranes have been used as electrodes in processes and devices for the generation of chlorine and hydrogen by electrolysis of an aqueous alkali metal halide and for the electrolysis of water. The catalytic electrodes at which the chlorine and hydrogen are produced are thin, porous, gas permeable catalytic electrodes which are bonded to or embedded in opposite surfaces of the membrane so that the chlorine and hydrogen are generated (substantially) at the electrode membrane interfaces. This results in electrodes which have very low overvoltages for chlorine and hydrogen discharge.
It is desired to provide a solid polymer electrolyte electrolytic cell having a structural frame which would minimize the corrosion problems and would increase the relatively short useful life attendant with those frames used by the prior art.